JP3875878B2

JP3875878B2 - Spot-welding method for high-strength steel sheets with excellent fatigue strength characteristics of welds

Info

Publication number: JP3875878B2
Application number: JP2001354269A
Authority: JP
Inventors: 初彦及川; 薫川崎; 誠司古迫; 隆田中; 順一小林
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2000-11-22
Filing date: 2001-11-20
Publication date: 2007-01-31
Anticipated expiration: 2021-11-20
Also published as: JP2002219576A

Description

【０００１】
【発明の属する技術分野】
本発明は、主に自動車用部品および車体などの組立に用いられる高強度鋼板のスポット溶接方法に関し、詳しくは、溶接部の疲労強度特性に優れた高強度鋼板のスポット溶接方法に関するものである。
【０００２】
【従来の技術】
近年、自動車の低燃費化、ＣＯ₂排出量削減および衝突安全性向上等の対策のため、自動車分野等では、自動車の車体や部品などに薄肉の高強度鋼板を使用するニーズが高まっている。
【０００３】
一方、自動車の車体や部品などの組立にはスポット溶接方法が主に用いられているが、高強度鋼板をスポット溶接法で溶接する場合には、以下のような問題がある。
【０００４】
一般的に、スポット溶接継手の品質指標としては、引張強さと共に疲労強度が重要となる。通常、自動車の継手では、せん断方向に力がかかるように設計される場合が多い。しかし、溶接継手のせん断方向の引張強さ（引張せん断強さ）は鋼板の引張強さとともに増加するが、せん断方向に負荷された場合の疲労強度は鋼板の引張強さが増加してもほとんど増加しない。例を上げるなら、引張強さが２９０ＭＰａの軟鋼板の代わりに、引張強さが５９０ＭＰａの高強度鋼板を用いれば、スポット溶接継手の引張せん断強さ（溶接継手のせん断方向に引張荷重を負荷した場合の引張強さ）はほぼ２倍になるが、溶接継手のせん断方向に繰り返し荷重を負荷した場合の疲労強度、例えば、応力負荷の回数が２×１０⁶回における荷重を疲労強度と定義すると、疲労強度は増加せず軟鋼板の場合とほぼ同じ値を示すのである。このように、疲労強度が低い値を示す原因としては、従来、報告されているように、スポット溶接部のノッチ形状が考えられる。すなわち、図１で示したように、鋼板１の間に存在するナゲット２の部分がノッチ形状になっているため、引張せん断方向（矢印方向）３に荷重を負荷して疲労試験を行った場合、引張強さの高い鋼板を用いても、このノッチ効果によって疲労強度が向上しないのである。特に、高強度鋼板を用いた場合には、軟鋼板を用いた場合に比べて、ナゲット部の硬さが増加するため、このノッチ効果は顕著になる。一方、溶接継手の剥離方向の引張強さ（十字引張強さ）は、鋼板の引張強さの増加とともにわずかに増加するが、剥離方向（引張せん断方向（矢印方向）３と垂直な方向）に荷重を負荷して疲労試験を行った場合には、高強度鋼板の溶接継手の疲労強度は、鋼板の引張強さとともに増加せず軟鋼と同じであり、この場合は、ナゲット周辺部での応力集中が顕著であり、局部の応力負荷が高まってそこでクラックが発生しやすくなるため、引張せん断方向に繰り返し荷重を負荷した場合に比べて疲労強度は一桁程度低下する。
【０００５】
一般に、鋼板の引張強さが増加するほど、下記▲１▼式、▲２▼式で示される炭素当量ＣｅｑｈとＣｅｑｔの値が高くなる傾向にある。Ｃｅｑｈは溶接部の硬さに対応する炭素当量であり、また、Ｃｅｑｔは溶接部の靭性に対応する炭素当量である。
Ｃｅｑｈ＝Ｃ＋Ｓｉ／４０＋Ｃｒ／２０（％） ▲１▼
Ｃｅｑｔ＝Ｃ＋Ｓｉ／３０＋Ｍｎ／２０＋２Ｐ＋４Ｓ（％） ▲２▼
（式中、Ｃ、Ｓｉ、Ｃｒ、Ｍｎ、Ｐ、Ｓは、それぞれ鋼中の炭素、珪素、クロム、マンガン、リン、硫黄の各含有量（質量％）を示す）
【０００６】
高強度鋼板では、引張強さの増加とともに炭素当量ＣｅｑｈとＣｅｑｔが高くなるため、引張強さが高い高強度鋼板ほどスポット溶接（ナゲット）部と熱影響部の硬さが高くなり、また、靱性が低下して、破壊が容易に起こりやすくなる。
【０００７】
以上の理由で高強度鋼板のスポット溶接部の疲労強度は、高強度鋼板の引張強さが増加しても増加せずに、軟鋼の場合と同じ程度になると考えられる。
【０００８】
従来の高強度鋼板のスポット溶接における溶接継手の疲労強度を向上させる手段としては、例えば、「鉄と鋼」第６８巻（１９８２年）第９号第１４４４ページ〜第１４５１ページにあるように、スポット溶接の通電が完了した後の一定時間経過後にテンパー通電を行うことによりスポット溶接（ナゲット）部と熱影響部を焼鈍して硬さを低下させ、残留応力を変化させる方法が知られている。しかし、この方法は、テンパー通電の適正な条件範囲の幅が非常に狭く、また、操業条件の変化により再現性が乏しいという実用上の問題がある。特に、めっき鋼板を連続的に打点してスポット溶接する場合には、打点数の増加とともに電極先端がめっきとの合金化反応によって劣化し、電極先端径が増大して、電流密度が低下し最適なテンパー通電条件から外れるため、安定的に継手の疲労強度を向上させることが困難となる。
【０００９】
スポット溶接部の疲労強度を向上させる手段としては、これ以外にも、特昭６３−３１７６２５、特平２−１６３３２３、特平５−２６３１８４、特平９−２６８３４６、特平１０−８１８７、特平１１−２７９６８９公報などに開示されているように、疲労強度特性が優れた鋼板を用いてスポット溶接する方法が知られているが、これらは軟鋼板のスポット溶接に関するものであり、高強度鋼板のスポット溶接部の疲労強度を向上させる方法については、未だ報告された例はない。
【００１０】
【発明が解決しようとする課題】
上記のように、高強度鋼板を用いてスポット溶接を行う場合の溶接継手の疲労強度は、軟鋼板を用いた場合と変わらない。車体を軽量化するためには、高強度鋼板の板厚を低減させることが必要となるが、このように、高強度鋼板溶接部の疲労強度が軟鋼板を用いた場合と同じであるならば、板厚を低減させることによって疲労強度が低下するため、板厚を低減させることが困難となる。その結果、自動車分野などで、高強度鋼板を用いることによる安全性向上や軽量化による低燃料費化、ＣＯ₂排出量削減のメリットを十分に享受することができない。
【００１１】
一方、高強度鋼板のスポット溶接部の疲労強度を向上させるためにスポット溶接打点数を増やす従来の方法は、作業効率の低下や、コスト上昇および設計自由度の制約などの問題がある。
【００１２】
本発明は、これらの従来の問題を解決するために、高強度鋼板のスポット溶接方法において良好な溶接作業性を確保しつつ溶接継手の疲労強度特性を向上することができる高強度鋼板のスポット溶接方法を提供することを目的とする。
【００１３】
【課題を解決するための手段】
本発明は、スポット溶接時の電極加圧力を電極先端径に合わせて所定範囲に調整する方法、スポット溶接後の電極保持時間を所定範囲に調整する方法、および被溶接材としてフェライト中に残留オーステナイトを含んだ加工誘起変態型複合組織鋼を用いそのマルテンサイト変態の体積膨張を利用する方法を用いることにより溶接部に圧縮残留応力を発生させ、スポット溶接継手の疲労強度を向上させることを特徴とするものであり、すなわち、本発明の要旨とするところは、以下の通りである。
（１）高強度鋼板のスポット溶接において、被溶接材として引張強さＴＳが４３０〜１３００ＭＰａの範囲内の高強度鋼板を用い、スポット溶接時の電極加圧力Ｐを下記（１）式および（４）式を満たすように調整して溶接し、かつスポット溶接後の電極保持時間ＨＴを下記（３）式を満たすように調整することを特徴とする高強度鋼板のスポット溶接方法。
０．１３４×ｔ×ＴＳ^1/2 ≦Ｐ≦０．１７０×ｔ×ＴＳ^1/2 （ｋＮ）（１）
１３０−１６０×ｔ＋２１０×ｔ ² ≦ＨＴ（ｍｓ）（３）
４×ｔ^1/2 ≦ｄ≦６．５×ｔ^1/2 （ｍｍ）（４）
但し、ｔ：被溶接材の厚み（ｍｍ）、ＴＳ：被溶接材の引張強さ（ＭＰａ）、Ｐ：溶接電極の加圧力（ｋＮ）、ｄ：溶接電極の先端径（ｍｍ）、ＨＴ：溶接後の電極保持時間（ｍｓ）
（２）高強度鋼板のスポット溶接において、被溶接材として引張強さＴＳが４３０〜１３００ＭＰａの範囲内の高強度鋼板を用い、スポット溶接時の電極加圧力Ｐを下記（２）式および（４）式を満たすように調整して溶接し、かつスポット溶接後の電極保持時間ＨＴを下記（３）式を満たすように調整することを特徴とする高強度鋼板のスポット溶接方法。
0.00510×ＴＳ^1/2 ×ｄ² ≦Ｐ≦0.00645×ＴＳ^1/2 ×ｄ²（ｋＮ）（２）
１３０−１６０×ｔ＋２１０×ｔ ² ≦ＨＴ（ｍｓ）（３）
４×ｔ^1/2 ≦ｄ≦６．５×ｔ^1/2 （ｍｍ）（４）
但し、ＴＳ：被溶接材の引張強さ（ＭＰａ）、ｄ：溶接電極の先端径（ｍｍ）、Ｐ：溶接電極の加圧力（ｋＮ）、ｔ：被溶接材の厚み（ｍｍ）、ＨＴ：溶接後の電極保持時間（ｍｓ）
（３）前記高強度鋼板が、組織中に残留オーステナイトを含有する加工誘起変態型複合組織鋼板であることを特徴とする上記（１）又は（２）に記載の高強度鋼板のスポット溶接方法。
【００１４】
【発明の実施の形態】
本発明者らは、まず、高強度鋼板をスポット溶接した継手の疲労強度を向上させる方法として、▲１▼溶接金属（ナゲット）端部のノッチ形状を変えて応力集中が起こり難い形状にする、▲２▼溶接金属（ナゲット）部とその周辺の熱影響（ＨＡＺ）部の硬さを低下させる、▲３▼溶接部に圧縮残留応力を発生させることにより相対的に引張残留応力を低減させる、の大きく３つの方法について検討した。
【００１５】
▲１▼の方法については、例えば、「鉄と鋼」第６８巻（１９８２年）第９号第１４４４ページ〜第１４５１ページにあるように、意図的に電流を上げて溶接通電により溶接金属に散り（鋼板間の溶接金属（ナゲット）端部から溶融金属が吹き飛ぶ現象）を発生させて、溶接金属（ナゲット）端部の形状を変化させる方法が知られている。しかし、この方法では、溶接金属（ナゲット）端部の形状がばらつき、実際、疲労強度もかなりばらつくことが知られている。
【００１６】
▲２▼の溶接金属および熱影響（ＨＡＺ）部の硬さや引張残留応力を低減させる方法としては、上述のように、溶接完了後の冷却後に溶接電極から溶接部に一定時間通電する（後通電）ことにより溶接部をテンパー処理する方法が従来知られている。
【００１７】
しかし、この方法は、既に述べたように溶接部のテンパー処理のための最適通電条件の範囲が非常に狭く、また、操業条件の変化などにより再現性が乏しいという問題がある。
【００１８】
一方、発明者らは、▲３▼の溶接部の引張残留応力を低減させるための圧縮残留応力の発生方法として、溶接時の電極加圧力と溶接金属（ナゲット）部と熱影響（ＨＡＺ）部のマルテンサイト変態による体積膨張を利用する方法が有効であると考え、実験などの詳細な検討を行った。
【００１９】
その結果、スポット溶接時の電極加圧力を電極先端径に合わせて被溶接材の厚みと引張強さに応じて調整する方法、スポット溶接後の電極保持時間を被溶接材の厚みに応じて調整する方法、および被溶接材としてフェライト中に残留オーステナイトを含んだ加工誘起変態型複合組織鋼を用いそのマルテンサイト変態の体積膨張を利用する方法を用いることにより溶接部に圧縮残留応力を発生させ、溶接継手の疲労強度を効果的に向上できることを見出した。
【００２０】
本発明は、これらの知見をもとになされたものであるが、以下に本発明の詳細を説明する。
【００２１】
図２は、本発明に係るスポット溶接方法を説明するための図である。本発明のスポット溶接方法では、被接合材である２枚の高強度鋼板１を重ね合わせ、その重ね合わせ部を銅製の溶接電極４で加圧しながら溶接通電し、２枚の高強度鋼板１の間に溶融金属部を形成させる。この溶融金属部は溶接通電終了後、主に水冷された電極による抜熱によって冷却されて凝固し、２枚の高強度鋼板１の間に溶接ナゲット（溶接金属）２が形成される。
【００２２】
高強度鋼板のスポット溶接後の溶接金属部とその周辺の熱影響（ＨＡＺ）部は、凝固、冷却過程でマルテンサイト変態する際に体積膨張が起きるが、その後、更に室温までの冷却過程で熱収縮が起こり最終的に形成される溶接部には、引張残留応力が導入される。この引張残留応力は、高強度鋼板のスポット溶接継手で疲労強度が低下する原因のひとつとして考えられている。
【００２３】
本発明の第１の発明では、被溶接材（図２に示す高強度鋼板１）として引張強さＴＳが４３０〜１３００ＭＰａの範囲内の高強度鋼板を用い、図２に示すようにスポット溶接を行う場合に、スポット溶接時の加圧力５を被溶接材の板厚ｔおよび引張強さＴＳに応じて下記（１）式の範囲内に調整し、スポット溶接部（ナゲット近傍）に圧縮残留応力を導入して、溶接継手の疲労強度を向上させる。
０．１３４×ｔ×ＴＳ^1/2≦Ｐ≦０．１７０×ｔ×ＴＳ^1/2（ｋＮ）（１）
但し、ｔ：被溶接材の厚み（ｍｍ）、ＴＳ：被溶接材の引張強さ（ＭＰａ）、Ｐ：溶接電極の加圧力（ｋＮ）
【００２４】
本発明で、被溶接材として用いる高強度鋼板の引張強さＴＳを４３０〜１３００ＭＰａの範囲に規定した理由は、引張強さが４３０ＭＰａより低い高強度鋼板を用いてスポット溶接すると、溶接継手の引張強さの向上効果が小さく、高強度化による安全性向上や軽量化による低燃費化、ＣＯ₂排出量削減のメリットを十分に享受することができないとともに、上記で述べた炭素当量ＣｅｑｈとＣｅｑｔの増加に起因した溶接部の硬さ上昇と靭性低下による溶接継手の疲労強度低下の影響が少ないからである。一方、引張強さが１３００ＭＰａより高い高強度鋼板を用いてスポット溶接すると、炭素当量ＣｅｑｈとＣｅｑｔの増加に起因した溶接部の硬さ上昇と靭性低下により、溶接継手の疲労強度向上の効果が認められないからである。引張強さＴＳの下限を５４０ＭＰａとすれば、溶接継手において、より良好な引張強さの向上と疲労強度の改善を得ることができる。また、引張強さの上限を１０００ＭＰａとすれば、溶接継手における、より一層の疲労強度向上効果を得ることができる。
【００２５】
スポット溶接時の溶接電極の加圧力Ｐを、上記（１）式で規定する理由は、上記（１）式の下限値より低い溶接電極の加圧力では、溶接部に十分な圧縮残留応力が導入されないため、溶接継手の疲労強度の向上効果がほとんど認められないからである。一方、上記（１）式の上限値より高い溶接電極の加圧力では、溶接時に鋼板が変形して加圧部表面に大きな圧痕を生じて外観形状を悪化させ、加圧部の板厚が薄くなり溶接継手の静的強度や疲労強度を低下させるという問題が生じるからである。
【００２６】
図３は、上記（１）式の導出の根拠となる発明者らの実験結果の一例として、引張強さＴＳが５９３ＭＰａの高強度鋼板を先端径が５×ｔ^1/2（ｍｍ）の電極を用いてスポット溶接する際の溶接電極の加圧力Ｐおよび板厚ｔと溶接継手の疲労強度との関係を示したものである。なお、図中の○印および×印は、溶接継手の疲労強度の評価結果を示すものであり、溶接継手の疲労強度が、引張強さＴＳ：３００ＭＰａの軟鋼板溶接時の疲労強度に対して２０％以上向上したものを○、向上しろが２０％未満のものを×で示した。図３から引張強さＴＳ：５９３ＭＰａの高強度鋼板をスポット溶接する際には、溶接電極の加圧力Ｐを高強度鋼板の引張強さＴＳ＝５９３（ＭＰａ）と板厚さｔ（ｍｍ）との関係から０．１３４×（５９３）^1/2×ｔ以上、０．１７０×（５９３）^1/2×ｔ以下の範囲内に設定することにより疲労強度が良好な溶接継手を得られることがわかる。
【００２７】
本発明における上記（１）式は、実験により種々の引張強さの高強度鋼板について、図３に示すようなスポット溶接時の溶接電極の加圧力Ｐおよび板厚ｔと溶接継手の疲労強度との関係を、電極先端径ｄが５×ｔ^1/2（ｍｍ）の電極において調査し求めたものである。
【００２８】
一方、本発明において、電極先端径ｄは下記（４）式で規定する範囲とすると好ましい。
４×ｔ^1/2≦ｄ≦６．５×ｔ^1/2（ｍｍ）（４）
その理由は、上記（４）式より低い値になると、溶融径が電極先端径を越えて散りが発生しやすくなるため十分な継手強度を得るための十分なナゲット径の値が得られなくなるからであり、また、面圧が高くなりすぎて、加圧部表面に圧痕を生じさせて外観形状を悪化させたり、加圧部の板厚が薄くなって溶接継手の静的強度や疲労強度を低下させるからである。したがって、下限値としては、４×ｔ^1/2（ｍｍ）程度が好ましい。一方、電極先端径ｄの値が上記（４）式の値より高い場合には、面圧が低下して溶接部に十分な圧縮残留応力が導入されず、溶接継手の疲労強度の向上効果がほとんど認められないからであり、また、電流密度が低下して、同じナゲット径を得るのにより高い電流を必要とするからである。また、フランジ幅には限界があるため、電極先端径をあまり大きくすることは出来ない。したがって、上限値としては、６．５×ｔ^1/2（ｍｍ）程度が好ましい。
【００２９】
なお、電極形状としては、ＪＩＳＣ９３０４に規定されているように、Ｆ型、Ｒ型、Ｄ型、ＤＲ型、ＣＦ型、ＣＲ型、ＥＦ型、ＥＲ型、Ｐ型があり、ＤＲ型、ＣＦ型、ＣＲ型、ＥＦ型、ＥＲ型、Ｐ型では電極先端径を特定できるが、他の電極では特定できないため、その場合には、鋼板との実質的な接触径を電極先端径とすれば良い。なお、ＤＲ型、ＣＦ型、ＣＲ型、ＥＲ型、Ｐ型でも、連続打点とともに電極先端径が増大するため、その場合には、鋼板との実質的な接触径を電極先端径と考えれば良い。
【００３０】
第一の発明では、上記のように被溶接材の引張強さとスポット溶接時の溶接電極の加圧力を規定することにより、溶接部近傍に圧縮残留応力を発生させ、溶接継手の疲労強度を向上させることができる。電極加圧力以外のスポット溶接時の溶接条件、例えば、溶接時の溶接電流および溶接時間などは、一般のスポット溶接条件に準ずれば良く、特に規定する必要はない。
【００３１】
ところで、溶接部に十分な圧縮残留応力を導入させるためには、電極で加圧している部分の面圧をある程度高く設定することが必要である。しかし、面圧をあまり高く設定すると、溶接部が変形してその部分の板厚が減少するため、継手の静的強度や疲労強度が低下する。したがって、適度な面圧の値に設定することが重要である。面圧は電極先端径ｄと電極加圧力Ｐとの関係で決まるため、面圧を適度な値に設定するためには、この関係を考慮する必要性がある。第２の発明では、電極先端径ｄと電極加圧力Ｐの関係を調査した結果、加圧力Ｐの適正範囲を電極先端径ｄの関数とすることで、上記第１の発明より一層疲労強度が向上することを見出した。
【００３２】
即ち、本発明の第２の発明では、高強度鋼板のスポット溶接において、被溶接材として引張強さＴＳが４３０〜１３００ＭＰａの範囲内の高強度鋼板を用い、溶接電極として先端径ｄの電極を用い、スポット溶接時の電極加圧力Ｐを下記（２）式を満たすように調整することにより、高強度鋼板のスポット溶接時の溶接部近傍に圧縮残留応力を導入して溶接継手の疲労強度を向上させるものである。
0.00510×ＴＳ^1/2×ｄ²≦Ｐ≦0.00645×ＴＳ^1/2×ｄ²（ｋＮ）（２）
但し、ＴＳ：被溶接材の引張強さ（ＭＰａ）、ｄ：溶接電極の先端径（ｍｍ）、Ｐ：溶接電極の加圧力（ｋＮ）
【００３３】
本発明で、被溶接材として用いる高強度鋼板の引張強さＴＳを４３０〜１３００ＭＰａの範囲に規定した理由は、引張強さが４３０ＭＰａより低い高強度鋼板を用いてスポット溶接すると、溶接継手の静的強度の向上効果が小さく、高強度化による安全性向上や軽量化による低燃費化、ＣＯ₂排出量削減のメリットを十分に享受することができないとともに、上記で述べた炭素当量ＣｅｑｈとＣｅｑｔの増加に起因した溶接部の硬さ上昇と靭性低下による溶接継手の疲労強度低下の影響が少ないからである。一方、引張強さが１３００ＭＰａより高い高強度鋼板を用いてスポット溶接すると、炭素当量ＣｅｑｈとＣｅｑｔの増加に起因した溶接部の硬さ上昇と靱性低下により、溶接継手の疲労強度向上の効果が認められないからである。第２の発明では、電極加圧力を電極先端径に応じて最適化しているので、圧縮残留応力がより有効に導入されて、疲労強度が向上する鋼板の引張強さＴＳの範囲を確実に拡大することができる。
【００３４】
本発明において、上記第１の発明と同様、電極先端径ｄを上記（４）式で規定する範囲とすると好ましい。その理由は、電極先端径ｄが上記（４）式の下限値より低い値になると、溶融径が電極先端径を越えて散りが発生しやすくなるため、十分な継手強度を得るための十分なナゲット径の値が得られなくなるからである。一方、電極先端径ｄの値が上記（４）式の上限値より高い値になると、電流密度が低下して、同じナゲット径を得るのにより高い電流を必要とするからであり、また、フランジ幅には限界があるため、電極先端径をあまり大きくすることが出来ないからである。したがって、電極先端径の変化、すなわち、電極先端面積の変化に比例させて、面圧が同じになるように加圧力を変化させれば、溶接部に十分な圧縮残留応力を導入させることが可能になるのである。一般的に、より大きなナゲット径を得る場合には、溶融部の径が電極先端径を越えて散りが発生することがないように電極先端径を増加させるが、この時の電極先端径が（４）式の範囲内で変化する場合には、上記（２）式のように加圧力を増加させると好ましい結果を得ることができる。
【００３５】
本発明の第３の発明では、上記第１又は第２の発明において、スポット溶接後の電極保持時間を下記（３）式を満たすように調整することにより、高強度鋼板のスポット溶接時の溶接部近傍に圧縮残留応力をより有効に導入して溶接継手の疲労強度を向上させるものである。
１３０−１６０×ｔ＋２１０×ｔ²≦ＨＴ（３）
但し、ｔ：被溶接材の厚み（ｍｍ）、ＨＴ：溶接後の電極保持時間（ｍｓ）
【００３６】
本発明で、被溶接材として用いる高強度鋼板の引張強さＴＳを４３０〜１３００ＭＰａの範囲に規定した理由は、引張強さが４３０ＭＰａより低い高強度鋼板を用いてスポット溶接すると、溶接継手の静的強度の向上効果が小さく、高強度化による安全性向上や軽量化による低燃費化、ＣＯ₂排出量削減のメリットを十分に享受することができないとともに、上記で述べた炭素当量ＣｅｑｈとＣｅｑｔの増加に起因した溶接部の硬さ上昇と靭性低下による溶接継手の疲労強度低下の影響が少ないからである。一方、引張強さが１３００ＭＰａより高い高強度鋼板を用いてスポット溶接すると、炭素当量ＣｅｑｈとＣｅｑｔの増加に起因した溶接部の硬さ上昇と靱性低下により、溶接継手の疲労強度向上の効果が認められないからである。第３の発明では、下記で述べるように、溶接後の保持時間ＨＴをある時間以上に設定することにより、低温になるまで溶接部に加圧力がかかるため、圧縮残留応力がより有効に導入されて、疲労強度が向上する鋼板の引張強さＴＳの範囲を確実に拡大することができる。
【００３７】
本発明で、スポット溶接後の電極保持時間ＨＴを上記（３）式で規定する理由は、電極保持時間が上記（３）式の値より低い場合には、溶接部の温度が高くその部分の変形抵抗が低いために加圧力をかけても塑性変形が進み、溶接部近傍に十分な圧縮残留応力が導入されないため、溶接継手の疲労強度の更なる向上効果が認められないからである。スポット溶接時の加圧力Ｐを上記（１）式又は（２）式で規定する範囲に設定し、また、溶接後の電極保持時間ＨＴを上記（３）式で規定する範囲に設定することによって、温度が低下した状態でも加圧力が十分にかかるため、溶接部に有効に圧縮残留応力が導入され、疲労強度が更に向上するのである。また、溶接後の保持時間を規定することによって、１０５０〜１３００ＭＰａ範囲のスポット溶接部の疲労強度を確実に向上させることも可能となった。上記（３）式は、実験により種々の板厚の高強度鋼板について、スポット溶接後の電極保持時間ＨＴおよび板厚ｔと溶接継手の疲労強度との関係を調査し求めたものである。溶接後の保持時間ＨＴの上限については特に規定しないが、保持時間が長くなりすぎると、溶接電極が水冷されているため、溶接部の冷却速度が速くなって溶接部の硬さが上昇し、また、靱性が低下して、特に剥離方向の継手強度が低下するため望ましくはない。また、溶接終了までの時間が長くなって生産性が落ちる等の問題が生じるため、保持時間は必要最小限とするのが望ましく、例えば、５００〜８００ｍｓ程度の範囲に設定するのが好ましいものと考えられる。電極加圧力と保持時間以外の溶接条件、例えば、溶接時の電流、時間などは、一般のスポット溶接条件に準ずれば良く、特に規定する必要はない。
【００３８】
本発明で用いる被溶接材の板厚については、特に規定する必要がなく、一般的に自動車用部材および車体などで使われる鋼板の板厚、例えば、０．４ｍｍ〜３．２ｍｍ程度であれば本発明の効果を十分奏することができる。また、鋼板の種類については特に限定するものではなく、固溶型、析出型（Ｔｉ析出型、Ｎｂ析出型）、２相組織型（フェライト中にマルテンサイトを含む組織、あるいはフェライト中にベイナイトを含む組織）、加工誘起変態型（フェライト中に残留オーステナイトを含む組織）、などいずれのタイプの鋼板でも良い。材料の製造方法は、熱間圧延法でも冷間圧延法でも良く、裸鋼板でもめっき鋼板でも良い。被覆するめっきの種類は、導電性のものならいずれの種類（例えば、Ｚｎ、Ｚｎ−Ｆｅ、Ｚｎ−Ｎｉ、Ｚｎ−Ａｌ、Ｓｎ−Ｚｎ、など）であっても良いが、目付量は両面で１００／１００ｇ／ｍ²以下のものが望ましい。
【００３９】
本発明の第４の発明では、上記の第１〜第３の発明で用いる被溶接材として、フェライト中に残留オーステナイトを含有する加工誘起変態型複合組織鋼板を用いることにより、溶接電極の加圧力による溶接部近傍のマルテンサイト変態を促進させ、その変態時の体積膨張により圧縮残留応力を導入して溶接継手の疲労強度を向上させるものである。
【００４０】
加工誘起変態型複合組織鋼板は、フェライト中に数％以上の残留オーステナイトを含有し、鋼板の加工時に残留オーステナイトがマルテンサイトに変態することにより高い伸び特性が得られることが知られている。
【００４１】
本発明者らは、種々の実験等の検討から被溶接材として加工誘起変態型複合組織鋼板を用いて、スポット溶接時の溶接電極の加圧力を第１の発明で説明した上記（１）式又は第２の発明で説明した上記（２）式の範囲に設定した場合に、組織中に残留オーステナイトを含有しない他の鋼板に比べて、スポット溶接継手の疲労強度が向上することが明らかになった。
【００４２】
この疲労強度向上のメカニズムについては十分には明らかになっていないが、加工誘起変態型複合組織鋼板を用いた場合、スポット溶接部周囲の組織中に含まれる残留オーステナイトが溶接電極の加圧力によってマルテンサイト変態し、この変態による体積膨張によってナゲット近傍に弾性歪が蓄積されるため、最終的に高い圧縮残留応力が導入されるのではないかと推定される。従来、ナゲット近傍の熱影響部には引張残留応力が導入されるため、せん断方向に繰り返し荷重を負荷する疲労試験の場合には、この応力集中部で破壊が起こりやすかったが、本発明においてはナゲット近傍への圧縮残留応力の導入により引張残留応力の効果は抑制され、このため従来に比べて溶接継手の疲労強度が向上したものと考えられる。
【００４３】
【実施例】
以下に実施例により、本発明の効果を説明する。
【００４４】
（第１の実施例）
表１に示す、板厚が１．６ｍｍ、引張強さが３００〜１３７２ＭＰａの鋼板を用い、スポット溶接継手の疲れ試験方法（ＪＩＳＺ３１３８）に基づいて引張せん断疲労試験片（試験片形状：４０×１６０ｍｍ）を作製した。鋼板の品種は、軟鋼（記号：２９０Ｓ）、固溶強化型高強度鋼（記号：４４０Ｓ、５９０Ｓ）、析出強化型高強度鋼（記号：５９０Ｐ）、２相複合組織型高強度鋼（５９０Ｄ、７８０Ｄ、９８０Ｄ、１１８０Ｄ、１３７０Ｄ）である。その後、これらの試験片を、同鋼種・同板厚の組み合わせで重ね合わせ、電極先端径が６．５ｍｍの電極を用い、図２に示すナゲット２のナゲット径が６．３ｍｍになるような条件でスポット溶接を行って溶接継手を作製した。電極形状は、ＪＩＳＣ９３０４に規定するＣＲ型、先端曲率径は４０ｍｍ、電極材質はクロム銅である。また、表１に示す電極加圧力上限及び下限は、上記（１）式に基づいて計算したものである。
【００４５】
得られた溶接継手について、スポット溶接継手の疲れ試験方法（ＪＩＳＺ３１３８）に基づき疲労試験を実施した。表１に示す疲労強度は、疲労試験を応力比：０．０５、周波数：３０Ｈｚの条件で片振り試験を行った際の２×１０⁶回における疲労強度を示す。
【００４６】
表１に示す条件Ｎｏ．９の比較例は、溶接電極の加圧力は本発明の規定範囲内であるが、被溶接材である高強度鋼板の引張強さが本発明で規定する上限値：１３００ＭＰａを超えているため、溶接金属部の硬さが上昇するとともに靱性が低下し、その結果、溶接継手の疲労強度は低い値を示した。
【００４７】
条件Ｎｏ．１０〜１６の比較例は、溶接電極の加圧力が本発明の規定範囲より低いため、溶接金属部に十分な残留圧縮応力が導入されず、溶接継手の疲労強度は低い値を示した。
【００４８】
条件Ｎｏ．１７〜２３の比較例は、溶接電極の加圧力が本発明の規定範囲より高いため、加圧部表面に大きな圧痕を生じて外観形状が不良となり、また、加圧部の肉厚が薄くなり、溶接継手の疲労強度も低い値を示した。
【００４９】
また、条件Ｎｏ．８の比較例は、本発明の高強度鋼板のスポット溶接と比較するために、軟鋼をスポット溶接した例である。
【００５０】
一方、溶接電極の加圧力が本発明の規定範囲である条件Ｎｏ．１〜７の発明例は、いずれの鋼種においても、溶接継手の疲労強度は十分高くなり、加圧部表面の圧痕による外観形状が不良もなく良好な溶接継手が得られた。
【００５１】
なお、発明者らは、板厚が１．６ｍｍ以外の鋼板でも同様の試験を実施したが、同様な結果が得られた。
【００５２】
【表１】

【００５３】
（第２の実施例）
表２、３に示す、板厚が１．６ｍｍ、引張強さが３００〜１３７２ＭＰａの鋼板を用い、スポット溶接継手の疲れ試験方法（ＪＩＳＺ３１３８）に基づいて引張せん断疲労試験片（試験片形状：４０×１６０ｍｍ）を作製した。鋼板の品種は、軟鋼（記号：２９０Ｓ）、固溶強化型高強度鋼（記号：４４０Ｓ、５９０Ｓ）、析出強化型高強度鋼（記号：５９０Ｐ）、２相複合組織型高強度鋼（５９０Ｄ、７８０Ｄ、９８０Ｄ、１１８０Ｄ、１３７０Ｄ）である。その後、これらの試験片を、同鋼種・同板厚の組み合わせで重ね合わせ、３種類の電極先端径（５．０、６．５、７．５ｍｍ）の電極を用い、図２に示すナゲット２のナゲット径が５．０、６．３ｍｍになるような条件でスポット溶接を行って溶接継手を作製した。電極形状は、ＪＩＳＣ９３０４に規定するＣＲ型、先端曲率径は４０ｍｍ、電極材質はクロム銅である。また、表２、３に示す電極加圧力上限及び下限は、上記（２）式に基づいて計算したものである。
【００５４】
得られた溶接継手について、スポット溶接継手の疲れ試験方法（ＪＩＳＺ３１３８）に基づき疲労試験を実施した。表２、３に示す疲労強度は、疲労試験を応力比：０．０５、周波数：３０Ｈｚの条件で片振り試験を行った際の２×１０⁶回における疲労強度を示す。
【００５５】
表２、３に示す条件Ｎｏ．９、３２、５５の比較例は、溶接電極の加圧力は本発明の規定範囲内であるが、被溶接材である高強度鋼板の引張強さが本発明で規定する上限値：１３００ＭＰａを超えているため、溶接金属部の硬さが上昇するとともに靱性が低下し、その結果、溶接継手の疲労強度は低い値を示した。
【００５６】
条件Ｎｏ．１０〜１６、３３〜３９、５６〜６２の比較例は、溶接電極の加圧力が本発明の規定範囲より低いため、溶接金属部に十分な残留圧縮応力が導入されず、溶接継手の疲労強度が低い値を示した。
【００５７】
条件Ｎｏ．１７〜２３、４０〜４６、６３〜６９の比較例は、溶接電極の加圧力が本発明の規定範囲より高いため、加圧部表面に大きな圧痕を生じて外観形状が不良となり、また、加圧部の肉厚が薄くなり、溶接継手の疲労強度も低い値を示した。
【００５８】
また、条件Ｎｏ．８、３１、５４の比較例は、本発明の高強度鋼板のスポット溶接と比較するために、軟鋼をスポット溶接した例である。
【００５９】
一方、溶接電極の加圧力が本発明の規定範囲である条件Ｎｏ．１〜７、２４〜３０、４７〜５３の発明例は、いずれの鋼種においても、溶接継手の疲労強度は十分高くなり、加圧部表面の圧痕による外観形状不良もなく良好な溶接継手が得られた。
【００６０】
なお、発明者らは、板厚が１．６ｍｍ板厚の鋼板でも同様の試験を実施したが、同様な結果が得られた。
【００６１】
【表２】

【００６２】
【表３】

【００６３】
（第３の実施例）
表４に示す、板厚が１．６ｍｍ、引張強さが３００〜１３７２ＭＰａの鋼板を用い、スポット溶接継手の疲れ試験方法（ＪＩＳＺ３１３８）に基づいて引張せん断疲労試験片（試験片形状：４０×１６０ｍｍ）を作製した。鋼板の品種は、軟鋼（記号：２９０Ｓ）、固溶強化型高強度鋼（記号：４４０Ｓ、５９０Ｓ）、析出強化型高強度鋼（記号：５９０Ｐ）、２相複合組織型高強度鋼（５９０Ｄ、７８０Ｄ、９８０Ｄ、１１８０Ｄ、１３７０Ｄ）である。その後、これらの試験片を、同鋼種・同板厚の組み合わせで重ね合わせ、３種類の電極先端径（５．０、６．５、７．５ｍｍ）の電極を用い、図２に示すナゲット２のナゲット径が５．０、６．３ｍｍになるような条件でスポット溶接を行って溶接継手を作製した。なお、保持時間は６００ｍｓとした。電極形状は、ＪＩＳＣ９３０４に規定するＣＲ型、先端曲率径は４０ｍｍ、電極材質はクロム銅である。また、表４に示す電極加圧力上限及び下限は、上記（２）式に基づいて計算したものである。
【００６４】
得られた溶接継手について、スポット溶接継手の疲れ試験方法（ＪＩＳＺ３１３８）に基づき疲労試験を実施した。表４に示す疲労強度は、疲労試験を応力比：０．０５、周波数：３０Ｈｚの条件で片振り試験を行った際の２×１０⁶回における疲労強度を示す。
【００６５】
表４に示す条件Ｎｏ．９、１８、２７の比較例は、溶接電極の加圧力、溶接後の保持時間は本発明の規定範囲内であるが、被溶接材である高強度鋼板の引張強さが本発明で規定する上限値：１３００ＭＰａを超えているため、溶接金属部の硬さが上昇するとともに靱性が低下し、その結果、溶接継手の疲労強度は低い値を示した。
【００６６】
条件Ｎｏ．８、１７、２６の比較例は、本発明の高強度鋼板のスポット溶接と比較するために、軟鋼をスポット溶接した例である。
【００６７】
一方、条件Ｎｏ．１〜７、１０〜１６、１９〜２５の発明例では、溶接電極の加圧力が本発明の上記（２）式の範囲内にあると同時に、溶接後の電極保持時間が本発明の上記（３）式の範囲内にある。そのため、いずれの電極径、鋼種においても、溶接電極の加圧力は本発明の規定範囲であるが溶接後の保持時間が上記（３）式の範囲からは外れる実施例１、実施例２の場合に比べて、溶接継手の疲労強度はより一層高い値を示し、加圧部表面の圧痕による外観形状が不良もなく極めて良好な溶接継手が得られた。
【００６８】
なお、発明者らは、板厚が１．６ｍｍ以外の鋼板でも同様の試験を実施したが、同様な結果が得られた。
【００６９】
【表４】

【００７０】
（第４の実施例）
表５に示す、板厚が０．８〜１．６ｍｍ、引張強さが５９４〜１３７４ＭＰａの加工誘起変態型複合組織高強度鋼板（記号：５９０Ｔ、７８０Ｔ、９８０Ｔ、１１８０Ｔ、１３８０Ｔ）と、比較用として、板厚が０．８〜１．６ｍｍ、引張強さが２９６〜３００ＭＰａの軟鋼板（記号：２９０Ｓ）、板厚が１．０、１．６ｍｍ、引張強さが５９２〜５９４ＭＰａである固溶強化型高強度鋼板（記号：５９０Ｓ）、析出強化型高強度鋼板（記号：５９０Ｐ）、および２相複合組織型高強度鋼板（５９０Ｄ）を用いて、スポット溶接継手の疲れ試験方法（ＪＩＳＺ３１３８）に基づいて引張せん断疲労試験片（試験片形状：４０×１６０ｍｍ）を作製した。
【００７１】
その後、これらの試験片を、同鋼種・同板厚の組み合わせで重ね合わせ、５種類の先端径（４．５〜７．５ｍｍ）の電極を用い、表５で示した条件で、図２で示すナゲット２のナゲット径が５×ｔ^1/2ｍｍ（ただし、ｔは板厚（ｍｍ））になるような条件（表５の条件）でスポット溶接を行って溶接継手を作製した。電極形状は、ＪＩＳＣ９３０４に規定するＣＲ型、先端曲率径は４０ｍｍ、電極材質はクロム銅である。また、表５に示す電極加圧力上限及び下限は、上記（２）式に基づいて計算したものである。
【００７２】
得られた溶接継手について、スポット溶接継手の疲れ試験方法（ＪＩＳＺ３１３８）に基づき疲労試験を実施した。表５に示す疲労強度は、疲労試験を応力比：０．０５、周波数：３０Ｈｚの条件で片振り試験を行った際の２×１０⁶回における疲労強度を示す。
【００７３】
表５に示す条件Ｎｏ．１〜４、７〜１０、１６〜１９、２２〜２５、３１〜３４、４０〜４３の発明例は、引張強さが５９４〜１１８６ＭＰａの加工誘起変態型複合組織高強度鋼板を用いた場合であるが、何れの条件においても、条件Ｎｏ．５、１１、２０、２６、３５、４４の比較例である軟鋼板を用いた場合に比べて、溶接継手の疲労強度が向上した。
【００７４】
表５に示す条件Ｎｏ．６、１２、２１、２７、３６、４５の比較例は、引張強さが１３７２〜１３７４ＭＰａの場合であり、引張強さが本発明の範囲より高いため、条件Ｎｏ．５、１１、２０、２６、３５、４４の比較例である軟鋼板の場合に比べて疲労強度は向上しなかった。
【００７５】
また、引張強さが５９２〜５９４ＭＰａの加工誘起変態型複合組織以外の鋼板を用いた場合（条件１３〜１５、２８〜３０、３７〜３９、４６〜４８）にも、軟鋼の場合（条件Ｎｏ．１１、２６、３５、４４）に比べて疲労強度は向上するが、加工誘起変態型複合組織鋼板を用いた場合（条件Ｎｏ．７〜１０、２２〜２５、３１〜３４、４０〜４３）の方がその疲労強度の向上代は大きかった。
【００７６】
表５に示す条件Ｎｏ．４０〜４３は、電極加圧力が本発明範囲内にあると同時に、溶接後の保持時間が本発明の上記（３）式を満たす範囲にある。その結果、溶接後の保持時間が（３）式の範囲外である本発明例（Ｎｏ．２２〜２５）に比較して、溶接継手の疲労強度はより一層高い値を示し、加圧部表面の圧痕による外観形状が不良もなく極めて良好な溶接継手が得られた。
【００７７】
【表５】

【００７８】
【発明の効果】
本発明は、主に自動車用部品および車体などの組立に用いられる高強度鋼板のスポット溶接方法において、良好な溶接作業性を確保しつつ溶接継手の疲労強度特性を向上させることができる。これにより自動車分野などでの高強度鋼板の適用による安全性向上や軽量化による低燃費化、ＣＯ₂排出量削減のメリットなどを十分享受でき、社会的な貢献は多大である。
【図面の簡単な説明】
【図１】スポット溶接部の疲労試験を説明するための断面図である。
【図２】本発明におけるスポット溶接方法を説明するための断面図である。
【図３】引張強さＴＳ：５９３ＭＰａの高強度鋼板をスポット溶接する際の溶接電極の加圧力Ｐ１および板厚さｔと溶接継手の疲労強度の評価結果（○、×）との関係を示すグラフ。
【符号の説明】
１高強度鋼板
２ナゲット
３負荷方向
４溶接電極
５加圧力[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a spot welding method for high-strength steel sheets mainly used for assembling automobile parts and vehicle bodies, and more particularly to a spot welding method for high-strength steel sheets excellent in fatigue strength characteristics of welds.
[0002]
[Prior art]
In recent years, low fuel consumption of automobiles, CO₂In order to take measures such as reducing emissions and improving collision safety, there is a growing need in the automotive field and the like to use thin high-strength steel sheets for automobile bodies and parts.
[0003]
On the other hand, spot welding methods are mainly used for assembling automobile bodies and parts. However, when high strength steel plates are welded by spot welding methods, there are the following problems.
[0004]
Generally, fatigue strength is important as well as tensile strength as a quality index of a spot welded joint. Usually, an automobile joint is often designed to apply a force in the shear direction. However, the tensile strength of the welded joint in the shear direction (tensile shear strength) increases with the tensile strength of the steel sheet, but the fatigue strength when loaded in the shear direction is almost constant even if the tensile strength of the steel sheet increases. Does not increase. For example, if a high strength steel plate having a tensile strength of 590 MPa is used instead of a mild steel plate having a tensile strength of 290 MPa, the tensile shear strength of the spot welded joint (a tensile load was applied in the shear direction of the weld joint). The tensile strength in the case is almost doubled, but the fatigue strength when the load is repeatedly applied in the shear direction of the welded joint, for example, the number of stress loads is 2 × 10⁶When the load at the time is defined as fatigue strength, the fatigue strength does not increase and shows almost the same value as in the case of a mild steel plate. As described above, the cause of the low fatigue strength may be the notch shape of the spot weld, as reported conventionally. That is, as shown in FIG. 1, the nugget 2 portion existing between the steel plates 1 has a notch shape, and therefore a fatigue test is performed with a load applied in the tensile shear direction (arrow direction) 3. Even if a steel plate having a high tensile strength is used, the fatigue strength is not improved by the notch effect. In particular, when a high-strength steel plate is used, the notch effect becomes significant because the hardness of the nugget portion increases as compared with the case where a mild steel plate is used. On the other hand, the tensile strength (cross tensile strength) in the peeling direction of the welded joint slightly increases as the tensile strength of the steel sheet increases, but in the peeling direction (the direction perpendicular to the tensile shear direction (arrow direction) 3). When a fatigue test is performed with a load applied, the fatigue strength of the welded joint of the high-strength steel plate does not increase with the tensile strength of the steel plate and is the same as that of mild steel. Concentration is conspicuous and local stress load increases and cracks are likely to occur there. Therefore, the fatigue strength is reduced by an order of magnitude compared to the case where repeated load is applied in the tensile shear direction.
[0005]
In general, as the tensile strength of the steel sheet increases, the carbon equivalent values Ceqh and Ceqt represented by the following formulas (1) and (2) tend to increase. Ceqh is a carbon equivalent corresponding to the hardness of the welded part, and Ceqt is a carbon equivalent corresponding to the toughness of the welded part.
Ceqh = C + Si / 40 + Cr / 20 (%) (1)
Ceqt = C + Si / 30 + Mn / 20 + 2P + 4S (%) (2)
(In the formula, C, Si, Cr, Mn, P, and S indicate the respective contents (mass%) of carbon, silicon, chromium, manganese, phosphorus, and sulfur in the steel)
[0006]
In high-strength steel sheets, the carbon equivalents Ceqh and Ceqt increase with increasing tensile strength. Therefore, the higher strength steel sheets with higher tensile strength, the higher the hardness of the spot welded (nugget) part and heat-affected zone, and the higher the toughness. Decreases and breakage easily occurs.
[0007]
For the above reasons, it is considered that the fatigue strength of the spot welded portion of the high-strength steel plate does not increase even when the tensile strength of the high-strength steel plate increases, and is the same as that of mild steel.
[0008]
As means for improving the fatigue strength of a welded joint in spot welding of a conventional high-strength steel plate, for example, as described in “Iron and Steel” Vol. 68 (1982) No. 9, page 1444 to page 1451, A method is known in which the temper energization is performed after a lapse of a certain time after the energization of the spot welding is completed, thereby annealing the spot welding (nugget) portion and the heat affected zone to reduce the hardness and changing the residual stress. . However, this method has a practical problem that the appropriate condition range of the temper energization is very narrow, and the reproducibility is poor due to a change in operating conditions. In particular, when spot welding is performed by continuously striking a plated steel sheet, the electrode tip deteriorates due to an alloying reaction with plating as the number of striking points increases, the electrode tip diameter increases, and the current density decreases, making it optimal. Therefore, it is difficult to stably improve the fatigue strength of the joint.
[0009]
As other means for improving the fatigue strength of the spot welded portion, JP 63-317625, JP 2163323, JP 5263184, JP 9-268346, JP 10-8187, JP As disclosed in Japanese Patent Application Laid-Open No. 11-279589 and the like, methods of spot welding using steel plates having excellent fatigue strength characteristics are known, but these are related to spot welding of mild steel plates, There has been no report yet on a method for improving the fatigue strength of a spot weld.
[0010]
[Problems to be solved by the invention]
As described above, the fatigue strength of the welded joint when spot welding is performed using a high-strength steel plate is the same as that when a mild steel plate is used. In order to reduce the weight of the car body, it is necessary to reduce the thickness of the high-strength steel plate, but if the fatigue strength of the welded portion of the high-strength steel plate is the same as when using a mild steel plate, Since the fatigue strength is reduced by reducing the plate thickness, it is difficult to reduce the plate thickness. As a result, in the automotive field, etc., safety is improved by using high-strength steel plates, fuel costs are reduced by weight reduction, CO₂The benefits of emission reduction cannot be fully enjoyed.
[0011]
On the other hand, the conventional method of increasing the number of spot welding points in order to improve the fatigue strength of the spot welded portion of the high-strength steel plate has problems such as a reduction in work efficiency, an increase in cost, and a restriction on design flexibility.
[0012]
In order to solve these conventional problems, the present invention is able to improve the fatigue strength characteristics of a welded joint while ensuring good welding workability in the spot welding method of a high strength steel sheet. It aims to provide a method.
[0013]
[Means for Solving the Problems]
  The present invention relates to a method for adjusting an electrode pressing force during spot welding to a predetermined range in accordance with an electrode tip diameter, a method for adjusting an electrode holding time after spot welding to a predetermined range, and residual austenite in ferrite as a material to be welded. It is characterized by improving the fatigue strength of spot welded joints by generating compressive residual stress in welds by using the method of utilizing the volume expansion of the martensitic transformation using a work-induced transformation type composite steel containing steel. That is, the gist of the present invention is as follows.
(1) In spot welding of a high-strength steel plate, a high-strength steel plate having a tensile strength TS in the range of 430 to 1300 MPa is used as a material to be welded, and the electrode pressure P during spot welding is expressed by the following formulas (1) and (4 ) Adjust and weld to satisfy the formula, andThe electrode holding time HT after spot welding is adjusted to satisfy the following formula (3).A spot welding method for high strength steel sheets.
  0.134 × t × TS^1/2 ≦ P ≦ 0.170 × t × TS^1/2 (KN) (1)
  130-160 × t + 210 × t ² ≤HT (ms) (3)
  4 x t^1/2 ≦ d ≦ 6.5 × t^1/2 (Mm) (4)
  Where, t: thickness of welded material (mm), TS: tensile strength of welded material (MPa), P: welding electrode pressure (kN), d: tip diameter of welding electrode (mm), HT: Electrode holding time after welding (ms)
(2) In spot welding of a high-strength steel plate, a high-strength steel plate having a tensile strength TS in the range of 430 to 1300 MPa is used as a material to be welded, and the electrode pressure P during spot welding is expressed by the following formula (2) and (4 ) Adjust and weld to satisfy the formula, andThe electrode holding time HT after spot welding is adjusted to satisfy the following formula (3).A spot welding method for high strength steel sheets.
  0.00510 × TS^1/2 Xd² ≦ P ≦ 0.00645 × TS^1/2 Xd²(KN) (2)
  130-160 × t + 210 × t ² ≤HT (ms) (3)
  4 x t^1/2 ≦ d ≦ 6.5 × t^1/2 (Mm) (4)
  However, TS: Tensile strength (MPa) of welding material, d: Tip diameter of welding electrode (mm), P: Pressure of welding electrode (kN), t: Thickness of welding material (mm), HT: Electrode holding time after welding (ms)
(3The above-mentioned (1), wherein the high-strength steel sheet is a work-induced transformation type composite structure steel sheet containing retained austenite in the structure.Or (2)The spot welding method of the high strength steel plate as described in 1.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
First, as a method for improving the fatigue strength of a joint obtained by spot welding a high-strength steel plate, the inventors changed the notch shape at the end of the weld metal (nugget) to make it less likely to cause stress concentration. (2) Decrease the hardness of the weld metal (nugget) part and the surrounding heat affected (HAZ) part, (3) Reducing the tensile residual stress relatively by generating compressive residual stress in the weld part, Three methods were studied.
[0015]
Regarding the method of (1), for example, as described in “Iron and Steel” Vol. 68 (1982) No. 9, pages 1444 to 1451, the current is intentionally increased and welding metal is applied by welding energization. There has been known a method of changing the shape of a weld metal (nugget) end by generating scattering (a phenomenon in which molten metal blows off from the end of the weld metal (nugget) between steel plates). However, with this method, it is known that the shape of the end of the weld metal (nugget) varies and, in fact, the fatigue strength varies considerably.
[0016]
As described in (2) above, as a method for reducing the hardness and tensile residual stress of the weld metal and the heat-affected (HAZ) part, as described above, the welding electrode is energized for a certain period of time after the welding is completed (post-energization). Thus, a method of tempering the welded portion is conventionally known.
[0017]
However, this method has a problem that the range of the optimum energization conditions for the tempering of the welded portion is very narrow as described above, and the reproducibility is poor due to changes in operating conditions.
[0018]
On the other hand, as a method of generating compressive residual stress for reducing the tensile residual stress of the welded portion of (3), the inventors have applied the electrode pressing force, the weld metal (nugget) portion, and the thermal effect (HAZ) portion during welding. We considered that the method using volume expansion due to martensitic transformation is effective, and conducted detailed studies such as experiments.
[0019]
As a result, the electrode pressure during spot welding is adjusted according to the thickness and tensile strength of the welded material according to the electrode tip diameter, and the electrode holding time after spot welding is adjusted according to the thickness of the welded material And using a method of utilizing the volume expansion of the martensitic transformation using a work-induced transformation type composite structure steel containing residual austenite in ferrite as a material to be welded, and generating a compressive residual stress in the weld zone, It has been found that the fatigue strength of welded joints can be effectively improved.
[0020]
The present invention has been made based on these findings. The details of the present invention will be described below.
[0021]
FIG. 2 is a diagram for explaining a spot welding method according to the present invention. In the spot welding method of the present invention, two high-strength steel plates 1 that are materials to be joined are overlapped, and welding is conducted while pressing the overlapped portion with a welding electrode 4 made of copper. A molten metal part is formed between them. This molten metal portion is cooled and solidified by heat removal mainly by water-cooled electrodes after welding energization, and a weld nugget (welded metal) 2 is formed between the two high-strength steel plates 1.
[0022]
The weld metal part after spot welding of high-strength steel sheet and the surrounding heat affected (HAZ) part undergo volume expansion when undergoing martensitic transformation in the solidification and cooling process, but then heat further in the cooling process to room temperature. Tensile residual stress is introduced into the weld that is contracted and finally formed. This tensile residual stress is considered as one of the causes of a decrease in fatigue strength in spot welded joints of high-strength steel sheets.
[0023]
In the first invention of the present invention, a high strength steel plate having a tensile strength TS in the range of 430 to 1300 MPa is used as a material to be welded (high strength steel plate 1 shown in FIG. 2), and spot welding is performed as shown in FIG. In the case of performing the spot welding, the pressure 5 at the time of spot welding is adjusted within the range of the following formula (1) according to the thickness t and tensile strength TS of the material to be welded, and the compressive residual stress is applied to the spot welded portion (nugget vicinity) To improve the fatigue strength of welded joints.
0.134 × t × TS^1/2≦ P ≦ 0.170 × t × TS^1/2(KN) (1)
Where t: thickness of welded material (mm), TS: tensile strength of welded material (MPa), P: pressure applied to welding electrode (kN)
[0024]
In the present invention, the reason why the tensile strength TS of the high-strength steel plate used as a material to be welded is defined in the range of 430 to 1300 MPa is that when spot welding is performed using a high-strength steel plate having a tensile strength lower than 430 MPa, the tensile strength of the welded joint Strength improvement effect is small, safety improvement by high strength and fuel efficiency reduction by weight reduction, CO₂The benefits of reduced emissions cannot be fully enjoyed, and the effects of reduced fatigue strength of welded joints due to increased hardness and toughness of welds due to the increase in carbon equivalents Ceqh and Ceqt described above are small. It is. On the other hand, when spot welding is performed using a high-strength steel plate having a tensile strength higher than 1300 MPa, the effect of improving the fatigue strength of the welded joint is recognized due to the increase in hardness and toughness of the weld due to the increase in carbon equivalents Ceqh and Ceqt. Because it is not possible. If the lower limit of the tensile strength TS is 540 MPa, better tensile strength improvement and fatigue strength improvement can be obtained in the welded joint. Further, if the upper limit of the tensile strength is set to 1000 MPa, a further effect of improving the fatigue strength in the welded joint can be obtained.
[0025]
The reason why the pressure P of the welding electrode at the time of spot welding is defined by the above equation (1) is that sufficient compressive residual stress is introduced into the welded portion when the welding electrode pressure is lower than the lower limit of the above equation (1). This is because the effect of improving the fatigue strength of the welded joint is hardly recognized. On the other hand, when the welding electrode pressure is higher than the upper limit value of the above formula (1), the steel plate is deformed during welding and a large indentation is generated on the surface of the pressurizing part to deteriorate the appearance shape, and the thickness of the pressurizing part is thin. This is because there arises a problem that the static strength and fatigue strength of the welded joint are lowered.
[0026]
FIG. 3 shows, as an example of the results of the experiments by the inventors as the basis for the derivation of the above equation (1), a high strength steel plate having a tensile strength TS of 593 MPa and a tip diameter of 5 × t.^1/2The relationship between the welding pressure P and plate | board thickness t at the time of spot welding using the electrode of (mm), and the fatigue strength of a welded joint is shown. In the figure, the ◯ mark and the X mark indicate the evaluation results of the fatigue strength of the welded joint, and the fatigue strength of the welded joint is the fatigue strength at the time of welding a mild steel sheet with a tensile strength TS: 300 MPa. Those that were improved by 20% or more were indicated by ○, and those that were improved by less than 20% were indicated by ×. From FIG. 3, when spot-welding a high-strength steel sheet having a tensile strength TS of 593 MPa, the pressure P of the welding electrode is set to the tensile strength TS of the high-strength steel sheet = 593 (MPa) and the thickness t (mm). 0.134 × (593)^1/2× t or more, 0.170 × (593)^1/2It can be seen that a welded joint with good fatigue strength can be obtained by setting within the range of xt or less.
[0027]
The above formula (1) in the present invention is obtained from the experimental results for high strength steel plates having various tensile strengths, and the welding electrode pressure P and plate thickness t during spot welding as shown in FIG. The electrode tip diameter d is 5 × t^1/2(Mm) was investigated and calculated | required in the electrode.
[0028]
On the other hand, in the present invention, the electrode tip diameter d is preferably in the range defined by the following formula (4).
4 x t^1/2≦ d ≦ 6.5 × t^1/2(Mm) (4)
The reason is that when the value is lower than the above formula (4), the melt diameter exceeds the electrode tip diameter and the scattering tends to occur, so that a sufficient nugget diameter value for obtaining sufficient joint strength cannot be obtained. In addition, the surface pressure becomes too high, causing indentations on the surface of the pressurizing part to deteriorate the appearance shape, and the plate thickness of the pressurizing part is reduced to reduce the static strength and fatigue strength of the welded joint. It is because it lowers. Therefore, the lower limit is 4 × t.^1/2(Mm) is preferable. On the other hand, when the value of the electrode tip diameter d is higher than the value of the above formula (4), the surface pressure is lowered and sufficient compressive residual stress is not introduced into the welded portion, and the effect of improving the fatigue strength of the welded joint is obtained. This is because it is rarely observed, and the current density is reduced and a higher current is required to obtain the same nugget diameter. In addition, since the flange width is limited, the electrode tip diameter cannot be increased too much. Therefore, the upper limit is 6.5 × t^1/2(Mm) is preferable.
[0029]
As specified in JIS C 9304, there are F type, R type, D type, DR type, CF type, CR type, EF type, ER type, P type, and DR type. The electrode tip diameter can be specified for CF type, CR type, EF type, ER type, and P type, but cannot be specified for other electrodes. In that case, the substantial contact diameter with the steel plate is used as the electrode tip diameter. It ’s fine. In addition, in the DR type, CF type, CR type, ER type, and P type, the electrode tip diameter increases with the continuous hitting point. In that case, the substantial contact diameter with the steel plate may be considered as the electrode tip diameter. .
[0030]
In the first invention, as described above, the tensile strength of the material to be welded and the applied pressure of the welding electrode at the time of spot welding are specified, thereby generating compressive residual stress in the vicinity of the weld and improving the fatigue strength of the welded joint. Can be made. The welding conditions during spot welding other than the electrode pressing force, for example, the welding current and welding time during welding, may be in accordance with general spot welding conditions and need not be specified.
[0031]
By the way, in order to introduce a sufficient compressive residual stress to the welded portion, it is necessary to set the surface pressure of the portion pressed by the electrode to be high to some extent. However, if the surface pressure is set too high, the welded portion is deformed and the thickness of the portion is reduced, so that the static strength and fatigue strength of the joint are lowered. Therefore, it is important to set an appropriate surface pressure value. Since the surface pressure is determined by the relationship between the electrode tip diameter d and the electrode pressure P, it is necessary to consider this relationship in order to set the surface pressure to an appropriate value. In the second invention, as a result of investigating the relationship between the electrode tip diameter d and the electrode pressing force P, the fatigue strength is further improved than that of the first invention by making the appropriate range of the pressing force P a function of the electrode tip diameter d. I found it to improve.
[0032]
That is, in the second invention of the present invention, in spot welding of a high-strength steel plate, a high-strength steel plate having a tensile strength TS in a range of 430 to 1300 MPa is used as a material to be welded, and an electrode having a tip diameter d is used as a welding electrode. By adjusting the electrode pressure P at the time of spot welding to satisfy the following formula (2), the fatigue strength of the welded joint can be improved by introducing compressive residual stress in the vicinity of the welded portion at the time of spot welding of the high strength steel plate. It is to improve.
0.00510 × TS^1/2Xd²≦ P ≦ 0.00645 × TS^1/2Xd²(KN) (2)
However, TS: Tensile strength of welded material (MPa), d: Tip diameter of welding electrode (mm), P: Pressure applied to welding electrode (kN)
[0033]
In the present invention, the reason why the tensile strength TS of the high-strength steel plate used as a material to be welded is defined in the range of 430 to 1300 MPa is that when spot welding is performed using a high-strength steel plate having a tensile strength lower than 430 MPa, The effect of improving the mechanical strength is small, the safety is improved by increasing the strength and the fuel efficiency is reduced by reducing the weight,₂The benefits of reduced emissions cannot be fully enjoyed, and the effects of reduced fatigue strength of welded joints due to increased hardness and toughness of welds due to the increase in carbon equivalents Ceqh and Ceqt described above are small. It is. On the other hand, when spot welding is performed using a high-strength steel sheet having a tensile strength higher than 1300 MPa, the effect of improving the fatigue strength of the welded joint is recognized due to the increase in hardness and toughness of the weld due to the increase in carbon equivalents Ceqh and Ceqt. Because it is not possible. In the second invention, since the electrode pressing force is optimized according to the electrode tip diameter, the compressive residual stress is more effectively introduced, and the range of the tensile strength TS of the steel sheet that improves the fatigue strength is reliably expanded. can do.
[0034]
In the present invention, similarly to the first invention, it is preferable that the electrode tip diameter d is in a range defined by the above equation (4). The reason for this is that when the electrode tip diameter d is lower than the lower limit of the above equation (4), the melt diameter exceeds the electrode tip diameter, and scattering tends to occur, so that it is sufficient to obtain sufficient joint strength. This is because the value of the nugget diameter cannot be obtained. On the other hand, if the value of the electrode tip diameter d is higher than the upper limit value of the above equation (4), the current density decreases, and a higher current is required to obtain the same nugget diameter. This is because the width of the electrode tip cannot be increased too much because the width is limited. Therefore, if the applied pressure is changed so that the surface pressure is the same in proportion to the change in the electrode tip diameter, that is, the change in the electrode tip area, sufficient compressive residual stress can be introduced into the weld. It becomes. In general, in order to obtain a larger nugget diameter, the electrode tip diameter is increased so that the melted portion diameter does not exceed the electrode tip diameter so that scattering does not occur. In the case of changing within the range of the formula (4), a preferable result can be obtained by increasing the pressing force as in the formula (2).
[0035]
In the third invention of the present invention, in the first or second invention described above, the electrode holding time after spot welding is adjusted so as to satisfy the following expression (3), whereby welding at the time of spot welding of a high-strength steel sheet: The compressive residual stress is more effectively introduced in the vicinity of the joint to improve the fatigue strength of the welded joint.
130-160 × t + 210 × t²≦ HT (3)
Where t: thickness of workpiece (mm), HT: electrode holding time after welding (ms)
[0036]
In the present invention, the reason why the tensile strength TS of the high-strength steel plate used as a material to be welded is defined in the range of 430 to 1300 MPa is that when spot welding is performed using a high-strength steel plate having a tensile strength lower than 430 MPa, The effect of improving the mechanical strength is small, the safety is improved by increasing the strength and the fuel efficiency is reduced by reducing the weight,₂The benefits of reduced emissions cannot be fully enjoyed, and the effects of reduced fatigue strength of welded joints due to increased hardness and toughness of welds due to the increase in carbon equivalents Ceqh and Ceqt described above are small. It is. On the other hand, when spot welding is performed using a high-strength steel sheet having a tensile strength higher than 1300 MPa, the effect of improving the fatigue strength of the welded joint is recognized due to the increase in hardness and toughness of the weld due to the increase in carbon equivalents Ceqh and Ceqt. Because it is not possible. In the third invention, as will be described below, by setting the holding time HT after welding to a certain time or longer, a pressure is applied to the welded portion until the temperature becomes low, so that the compressive residual stress is more effectively introduced. Thus, the range of the tensile strength TS of the steel sheet that improves the fatigue strength can be reliably expanded.
[0037]
In the present invention, the reason why the electrode holding time HT after spot welding is defined by the above formula (3) is that when the electrode holding time is lower than the value of the above formula (3), the temperature of the welded portion is high and This is because, since the deformation resistance is low, plastic deformation proceeds even when pressure is applied, and sufficient compressive residual stress is not introduced in the vicinity of the welded portion, so that the effect of further improving the fatigue strength of the welded joint is not recognized. By setting the pressure P at the time of spot welding to the range specified by the above formula (1) or (2), and setting the electrode holding time HT after welding to the range specified by the above formula (3) Even when the temperature is lowered, a sufficient pressure is applied, so that compressive residual stress is effectively introduced into the welded portion, and the fatigue strength is further improved. Moreover, it became possible to improve the fatigue strength of the spot welded part in the range of 1050 to 1300 MPa reliably by defining the holding time after welding. The above equation (3) is obtained by investigating the relationship between the electrode holding time HT and the plate thickness t after spot welding and the fatigue strength of the welded joint for high-strength steel plates having various thicknesses through experiments. The upper limit of the holding time HT after welding is not particularly specified, but if the holding time becomes too long, the welding electrode is water-cooled, so that the cooling rate of the welded portion increases and the hardness of the welded portion increases, Further, it is not desirable because the toughness is lowered and the joint strength in the peeling direction is lowered. In addition, since problems such as a long time until the end of welding and a decrease in productivity occur, it is desirable to keep the holding time to a minimum, for example, it is preferable to set in the range of about 500 to 800 ms. Conceivable. The welding conditions other than the electrode pressing force and the holding time, for example, the current and time during welding, may be in accordance with general spot welding conditions and need not be specified.
[0038]
The thickness of the material to be welded used in the present invention does not need to be specified in particular, and is generally the thickness of a steel plate used for automobile members and car bodies, for example, about 0.4 mm to 3.2 mm. The effects of the present invention can be sufficiently achieved. Further, the type of the steel plate is not particularly limited, and is a solid solution type, a precipitation type (Ti precipitation type, Nb precipitation type), a two-phase structure type (a structure containing martensite in ferrite, or bainite in ferrite). Any type of steel sheet may be used, such as a structure including a deformation-induced transformation type (a structure including residual austenite in ferrite). The manufacturing method of the material may be a hot rolling method or a cold rolling method, and may be a bare steel plate or a plated steel plate. The type of plating to be coated may be any type as long as it is conductive (for example, Zn, Zn—Fe, Zn—Ni, Zn—Al, Sn—Zn, etc.). 100 / 100g / m²The following are desirable:
[0039]
In the fourth invention of the present invention, as a material to be welded used in the first to third inventions described above, by using a work-induced transformation type composite steel sheet containing retained austenite in ferrite, the pressure applied to the welding electrode This promotes the martensitic transformation in the vicinity of the welded portion due to, and introduces compressive residual stress due to volume expansion during the transformation, thereby improving the fatigue strength of the welded joint.
[0040]
It is known that a work-induced transformation type composite structure steel sheet contains several percent or more of retained austenite in ferrite, and high elongation characteristics can be obtained by transforming the retained austenite to martensite during the processing of the steel sheet.
[0041]
The present inventors have used the above-described formula (1) described in the first invention for the pressure applied to the welding electrode during spot welding using a work-induced transformation type composite steel sheet as a material to be welded based on various experiments and the like. Or it becomes clear that the fatigue strength of the spot welded joint is improved when set in the range of the above-described formula (2) described in the second invention, as compared with other steel sheets not containing residual austenite in the structure. It was.
[0042]
The mechanism for improving this fatigue strength has not been fully clarified. However, when a work-induced transformation type composite steel sheet is used, the retained austenite contained in the structure around the spot weld is martensite by the pressure of the welding electrode. It is presumed that high compressive residual stress is finally introduced because elastic deformation is accumulated in the vicinity of the nugget due to site transformation and volume expansion due to this transformation. Conventionally, since tensile residual stress is introduced into the heat-affected zone near the nugget, in the case of a fatigue test in which a repeated load is applied in the shear direction, this stress-concentrated portion was likely to break, but in the present invention By introducing compressive residual stress in the vicinity of the nugget, the effect of tensile residual stress is suppressed, and it is considered that the fatigue strength of the welded joint is improved compared to the conventional case.
[0043]
【Example】
The effects of the present invention will be described below with reference to examples.
[0044]
(First embodiment)
Using a steel plate having a plate thickness of 1.6 mm and a tensile strength of 300 to 1372 MPa shown in Table 1, a tensile shear fatigue test piece (test piece shape: 40 ×) based on a fatigue test method for spot welded joints (JIS Z3138). 160 mm). The types of steel plates are mild steel (symbol: 290S), solid solution strengthened high strength steel (symbol: 440S, 590S), precipitation strengthened high strength steel (symbol: 590P), dual-phase composite structure type high strength steel (590D, 780D, 980D, 1180D, 1370D). Thereafter, these test pieces were superposed with the same steel type and the same plate thickness, and the electrode tip diameter was 6.5 mm, and the nugget 2 shown in FIG. 2 had a nugget diameter of 6.3 mm. Spot welding was performed to prepare a welded joint. The electrode shape is a CR type specified in JIS C9304, the tip curvature diameter is 40 mm, and the electrode material is chromium copper. Moreover, the upper limit and the lower limit of the electrode pressing force shown in Table 1 are calculated based on the above formula (1).
[0045]
About the obtained welded joint, the fatigue test was implemented based on the fatigue test method (JIS Z3138) of a spot welded joint. The fatigue strength shown in Table 1 is 2 × 10 when the fatigue test is performed in a single swing test under the conditions of stress ratio: 0.05 and frequency: 30 Hz.⁶Shows the fatigue strength in a round.
[0046]
Condition No. shown in Table 1 In the comparative example of 9, the applied pressure of the welding electrode is within the specified range of the present invention, but the tensile strength of the high-strength steel plate that is the material to be welded exceeds the upper limit of 1300 MPa specified by the present invention. As the hardness of the weld metal part increased, the toughness decreased, and as a result, the fatigue strength of the welded joint showed a low value.
[0047]
Condition No. In Comparative Examples 10 to 16, since the welding electrode pressure was lower than the specified range of the present invention, sufficient residual compressive stress was not introduced into the weld metal part, and the fatigue strength of the welded joint showed a low value.
[0048]
Condition No. In Comparative Examples 17 to 23, the pressure applied to the welding electrode is higher than the specified range of the present invention, so that a large indentation is generated on the surface of the pressurizing portion, resulting in a poor appearance, and the thickness of the pressurizing portion is reduced. The fatigue strength of the welded joint also showed a low value.
[0049]
In addition, Condition No. The comparative example 8 is an example in which mild steel is spot-welded for comparison with spot welding of the high-strength steel sheet of the present invention.
[0050]
On the other hand, the condition No. in which the applied pressure of the welding electrode is within the specified range of the present invention. In any of the steel types of Examples 1 to 7, the fatigue strength of the welded joint was sufficiently high, and a good welded joint was obtained with no defects in the external shape due to the indentation on the surface of the pressure part.
[0051]
In addition, although the inventors performed the same test also with steel plates other than 1.6 mm in plate thickness, the same result was obtained.
[0052]
[Table 1]

[0053]
(Second embodiment)
A steel plate having a thickness of 1.6 mm and a tensile strength of 300 to 1372 MPa shown in Tables 2 and 3 was used, and a tensile shear fatigue test piece (test piece shape: based on a fatigue test method for spot welded joints (JIS Z3138)). 40 × 160 mm). The types of steel plates are mild steel (symbol: 290S), solid solution strengthened high strength steel (symbol: 440S, 590S), precipitation strengthened high strength steel (symbol: 590P), dual-phase composite structure type high strength steel (590D, 780D, 980D, 1180D, 1370D). Thereafter, these test pieces were overlapped with a combination of the same steel type and the same plate thickness, and three types of electrode tip diameters (5.0, 6.5, 7.5 mm) were used, and the nugget 2 shown in FIG. Spot welding was performed under conditions such that the nugget diameter of the steel was 5.0 and 6.3 mm, and a welded joint was produced. The electrode shape is a CR type specified in JIS C9304, the tip curvature diameter is 40 mm, and the electrode material is chromium copper. Moreover, the upper limit and the lower limit of the electrode pressure shown in Tables 2 and 3 are calculated based on the above formula (2).
[0054]
About the obtained welded joint, the fatigue test was implemented based on the fatigue test method (JIS Z3138) of a spot welded joint. The fatigue strengths shown in Tables 2 and 2 are 2 × 10 when the fatigue test is performed in a single swing test under the conditions of stress ratio: 0.05 and frequency: 30 Hz.⁶Shows the fatigue strength in a round.
[0055]
Condition Nos. In Comparative Examples 9, 32, and 55, the welding electrode pressure is within the specified range of the present invention, but the tensile strength of the high-strength steel sheet as the welded material exceeds the upper limit defined by the present invention: 1300 MPa. Therefore, the hardness of the weld metal part increased and the toughness decreased, and as a result, the fatigue strength of the welded joint showed a low value.
[0056]
Condition No. In Comparative Examples 10 to 16, 33 to 39, and 56 to 62, since the welding electrode pressure is lower than the specified range of the present invention, sufficient residual compressive stress is not introduced into the weld metal part, and the fatigue strength of the welded joint Showed a low value.
[0057]
Condition No. In Comparative Examples 17-23, 40-46, 63-69, the welding electrode has a higher pressing force than the specified range of the present invention. The thickness of the pressed part was reduced, and the fatigue strength of the welded joint was also low.
[0058]
In addition, Condition No. Comparative examples 8, 31, and 54 are examples in which mild steel is spot-welded for comparison with spot welding of the high-strength steel sheet of the present invention.
[0059]
On the other hand, the condition No. in which the applied pressure of the welding electrode is within the specified range of the present invention. The invention examples 1 to 7, 24 to 30, and 47 to 53 have a sufficiently high fatigue strength of the welded joint in any of the steel types, and a good welded joint is obtained without appearance defects due to indentations on the surface of the pressure part. It was.
[0060]
In addition, although the inventors performed the same test also on the steel plate with a plate thickness of 1.6 mm, similar results were obtained.
[0061]
[Table 2]

[0062]
[Table 3]

[0063]
(Third embodiment)
Using a steel plate having a plate thickness of 1.6 mm and a tensile strength of 300 to 1372 MPa shown in Table 4, a tensile shear fatigue test piece (test piece shape: 40 ×) based on a fatigue test method for spot welded joints (JIS Z3138). 160 mm). The types of steel plates are mild steel (symbol: 290S), solid solution strengthened high strength steel (symbol: 440S, 590S), precipitation strengthened high strength steel (symbol: 590P), dual-phase composite structure type high strength steel (590D, 780D, 980D, 1180D, 1370D). Thereafter, these test pieces were overlapped with a combination of the same steel type and the same plate thickness, and three types of electrode tip diameters (5.0, 6.5, 7.5 mm) were used, and the nugget 2 shown in FIG. Spot welding was performed under conditions such that the nugget diameter of the steel was 5.0 and 6.3 mm, and a welded joint was produced. The holding time was 600 ms. The electrode shape is a CR type specified in JIS C9304, the tip curvature diameter is 40 mm, and the electrode material is chromium copper. Moreover, the upper limit and the lower limit of the electrode pressing force shown in Table 4 are calculated based on the above equation (2).
[0064]
About the obtained welded joint, the fatigue test was implemented based on the fatigue test method (JIS Z3138) of a spot welded joint. The fatigue strength shown in Table 4 is 2 × 10 when the fatigue test is performed in a single swing test under the conditions of stress ratio: 0.05 and frequency: 30 Hz.⁶Shows the fatigue strength in a round.
[0065]
Condition No. shown in Table 4 In Comparative Examples 9, 18, and 27, the welding electrode pressure and the retention time after welding are within the specified range of the present invention, but the tensile strength of the high-strength steel sheet as the welded material is specified by the present invention. Since the upper limit is over 1300 MPa, the hardness of the weld metal part is increased and the toughness is lowered. As a result, the fatigue strength of the welded joint is low.
[0066]
Condition No. Comparative examples 8, 17, and 26 are examples in which mild steel is spot-welded for comparison with spot welding of the high-strength steel sheet of the present invention.
[0067]
On the other hand, Condition No. In the inventive examples 1 to 7, 10 to 16, and 19 to 25, the pressure of the welding electrode is within the range of the above formula (2) of the present invention, and at the same time, the electrode holding time after welding is the above ( 3) Within the range of the formula. Therefore, in any of the electrode diameters and steel types, in the case of Example 1 and Example 2 in which the pressing force of the welding electrode is within the specified range of the present invention, but the holding time after welding is out of the range of the above formula (3). In comparison with the above, the fatigue strength of the welded joint showed a much higher value, and a very good welded joint was obtained without any appearance defect due to the indentation on the surface of the pressure part.
[0068]
In addition, although the inventors performed the same test also with steel plates other than 1.6 mm in plate thickness, the same result was obtained.
[0069]
[Table 4]

[0070]
(Fourth embodiment)
Table 5 shows a processing-induced transformation type composite structure high-strength steel sheet (symbol: 590T, 780T, 980T, 1180T, 1380T) having a thickness of 0.8 to 1.6 mm and a tensile strength of 594 to 1374 MPa, for comparison. A mild steel plate (symbol: 290S) having a plate thickness of 0.8 to 1.6 mm and a tensile strength of 296 to 300 MPa, a solid thickness of 1.0 and 1.6 mm, and a tensile strength of 592 to 594 MPa. Fatigue test method for spot welded joint (JIS Z3138) using melt strengthened high strength steel plate (symbol: 590S), precipitation strengthened high strength steel plate (symbol: 590P), and dual phase composite structure type high strength steel plate (590D). ) To prepare a tensile shear fatigue test piece (test piece shape: 40 × 160 mm).
[0071]
Thereafter, these test pieces were overlapped with a combination of the same steel type and the same plate thickness, and electrodes having five types of tip diameters (4.5 to 7.5 mm) were used, and the conditions shown in Table 5 were used in FIG. The nugget diameter of the nugget 2 shown is 5 × t^1/2Spot welding was performed under the conditions (conditions in Table 5) such that mm (where t is the plate thickness (mm)) to produce a welded joint. The electrode shape is a CR type specified in JIS C9304, the tip curvature diameter is 40 mm, and the electrode material is chromium copper. Moreover, the upper limit and the lower limit of the electrode pressing force shown in Table 5 are calculated based on the above equation (2).
[0072]
About the obtained welded joint, the fatigue test was implemented based on the fatigue test method (JIS Z3138) of a spot welded joint. The fatigue strength shown in Table 5 is 2 × 10 when the fatigue test is performed by a swing test under the conditions of stress ratio: 0.05 and frequency: 30 Hz.⁶Shows the fatigue strength in a round.
[0073]
Condition No. shown in Table 5 Inventive examples 1 to 4, 7 to 10, 16 to 19, 22 to 25, 31 to 34, and 40 to 43 are cases in which a work-induced transformation type composite structure high strength steel plate having a tensile strength of 594 to 1186 MPa is used. However, in any condition, the condition No. Compared with the case where the mild steel plate which is a comparative example of 5, 11, 20, 26, 35, and 44 was used, the fatigue strength of the welded joint was improved.
[0074]
Condition No. shown in Table 5 The comparative examples of 6, 12, 21, 27, 36, and 45 are cases where the tensile strength is 1372 to 1374 MPa, and the tensile strength is higher than the range of the present invention. Fatigue strength was not improved as compared with the case of a mild steel plate as a comparative example of 5, 11, 20, 26, 35, and 44.
[0075]
Further, when a steel plate other than the work-induced transformation type composite structure having a tensile strength of 592 to 594 MPa is used (conditions 13 to 15, 28 to 30, 37 to 39, 46 to 48), the case of mild steel (condition No. .11, 26, 35, 44), the fatigue strength is improved, but when a work-induced transformation type composite steel sheet is used (Condition Nos. 7 to 10, 22 to 25, 31 to 34, 40 to 43). The increase in fatigue strength was greater for.
[0076]
Condition No. shown in Table 5 In 40 to 43, the electrode pressing force is within the range of the present invention, and at the same time, the holding time after welding is in the range satisfying the above expression (3) of the present invention. As a result, the fatigue strength of the welded joint shows a higher value compared to the inventive examples (Nos. 22 to 25) in which the retention time after welding is outside the range of the formula (3), and the surface of the pressurizing part A very good welded joint was obtained with no defects in the external shape due to the indentation.
[0077]
[Table 5]

[0078]
【The invention's effect】
INDUSTRIAL APPLICABILITY The present invention can improve the fatigue strength characteristics of a welded joint while ensuring good welding workability in a spot welding method for high-strength steel sheets mainly used for assembling automobile parts and vehicle bodies. As a result, safety is improved by applying high-strength steel sheets in the automobile field, fuel efficiency is reduced by weight reduction, CO₂The benefits of reducing emissions can be fully enjoyed, making a significant social contribution.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view for explaining a fatigue test of a spot weld.
FIG. 2 is a cross-sectional view for explaining a spot welding method in the present invention.
FIG. 3 shows the relationship between the applied pressure P1 and thickness t of a welding electrode when spot-welding a high-strength steel sheet having a tensile strength TS of 593 MPa and the evaluation results (◯, x) of fatigue strength of welded joints. Graph.
[Explanation of symbols]
1 High-strength steel sheet
2 Nuggets
3 Load direction
4 Welding electrodes
5 Pressure

Claims

In spot welding of high-strength steel plates, high-strength steel plates having a tensile strength TS in the range of 430 to 1300 MPa are used as materials to be welded, and the electrode pressure P during spot welding is expressed by the following formulas (1) and (4): A spot welding method for high strength steel sheets, characterized by adjusting and welding so as to satisfy, and adjusting an electrode holding time HT after spot welding so as to satisfy the following expression (3) .
0.134 × t × TS ^1/2 ≦ P ≦ 0.170 × t × TS ^1/2 (kN) (1)
130-160 × t + 210 × t ² ≤HT (ms) (3)
4 × t ^1/2 ≦ d ≦ 6.5 × t ^1/2 (mm) (4)
However, t: Thickness (mm) of the material to be welded, TS: Tensile strength (MPa) of the material to be welded, P: Pressurizing force (kN) of the welding electrode, d: Tip diameter (mm) of the welding electrode , HT: Electrode holding time after welding (ms)

In spot welding of high-strength steel plates, high-strength steel plates having a tensile strength TS in the range of 430 to 1300 MPa are used as materials to be welded, and the electrode pressure P during spot welding is expressed by the following formulas (2) and (4): A spot welding method for high strength steel sheets, characterized by adjusting and welding so as to satisfy, and adjusting an electrode holding time HT after spot welding so as to satisfy the following expression (3) .
0.00510 × TS ^1/2 × d ² ≦ P ≦ 0.00645 × TS ^1/2 × d ² (kN) (2)
130-160 × t + 210 × t ² ≤HT (ms) (3)
4 × t ^1/2 ≦ d ≦ 6.5 × t ^1/2 (mm) (4)
However, TS: Tensile strength (MPa) of welding material, d: Tip diameter (mm) of welding electrode, P: Pressure of welding electrode (kN), t: Thickness (mm) of welding material , HT: Electrode holding time after welding (ms)

The high-strength steel plate spot welding method according to claim 1 or 2 , wherein the high-strength steel plate is a work-induced transformation type composite steel plate containing retained austenite in the structure.